He added he wasn't sure if the building had been completely empty when the blast went off.

Sven-Erik Olsson, from Stockholm Police, said: "When we reached the smoke from the room and the windows were shattered."

Stockholm's bomb squad are at the scene and police are investigating the incident.

A police spokesman said: "No one was inside, no one was injured.

"It had been locked since earlier in the evening."

No one has been arrested and there are currently no suspects, he added.

Twenty-eight people were killed and dozens wounded in Turkey's capital Ankara earlier on Wednesday when a car laden with explosives detonated next to military buses near the armed forces' headquarters, parliament and other government buildings.

We'll be bringing you the very latest updates, pictures and video on this breaking news story.